retain backbuffer after flip

BlitzMax Forums/OpenGL Module/retain backbuffer after flip

Warner

(Posted 2009) [#1]

I'm trying to achieve that the backbuffer is retained after Flip().
After searching various subjects, I came up with this:

SetGraphicsDriver GLMax2DDriver()

Graphics 800, 600, 0, 60, 2

Global buffer:TGLImageFrame = TGLImageFrame.CreateFromPixmap(CreatePixmap(800, 600, PF_RGBA8888), 0)

Repeat

	SetColor Rand(255),Rand(255),Rand(255)
	DrawOval Rand(800),Rand(600),10,10
	
	RetainFlip()
	
Until KeyHit(27)

End

Function RetainFlip()
	
	Local u0#=0
	Local v0#=0
	Local u1#=800.0/1024.0
	Local v1#=600.0/1024.0

	SetScale 1, 1
	SetRotation 0
	SetColor 255, 255, 255
	SetAlpha 1.0
	SetBlend SOLIDBLEND

	glRasterPos2i 0,0
	glBindtexture GL_TEXTURE_2D, buffer.name
	glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 0, 0, 1024, 1024, 0)

	Flip
		
	SetScale 1, 1
	SetRotation 0
	SetColor 255, 255, 255
	SetAlpha 1.0
	SetBlend SOLIDBLEND
	
	glRasterPos2i 0,0
	glBindTexture GL_TEXTURE_2D,buffer.name
	glEnable GL_TEXTURE_2D
			
	glBegin GL_QUADS
	glTexCoord2f u0,v1
	glVertex2f 0, 0
	glTexCoord2f u1,v1
	glVertex2f 800, 0
	glTexCoord2f u1,v0
	glVertex2f 800, 600
	glTexCoord2f u0,v0
	glVertex2f 0, 600
	glEnd

	glDisable GL_TEXTURE_2D
			
End Function

It does show the backbuffer content, except that it gets distorted by 1 pixel vertical, and 1 pixel horizontal. It doesn't make any sense to me.
I was wondering if this happens only on my system?

ImaginaryHuman

(Posted 2009) [#2]

Maybe a float accuracy issue? Try adding or subtracting 1 from the quad corner coordinates.

Warner

(Posted 2009) [#3]

Yes, a rounding issue would seem a logical explaination. When I try substracting 1, the smur goes towards the center. Well, I'll experiment some more, thanks.

Warner

(Posted 2009) [#4]

I tested it on another computer today, but then the code works normally.
This makes it more difficult to work around this. Has anybody tried this so far?

jsp	(Posted 2009) [#5]

Works fine here on an Intel Card.

Warner

(Posted 2009) [#6]

Hmm .. my onboard videocard must be buggy then.
Thanks for trying!

Scienthsine

(Posted 2009) [#7]

Try playing with/forcing different opengl texture wrap/clamp parameters. http://msdn.microsoft.com/en-us/library/dd368639%28VS.85%29.aspx

Warner

(Posted 2009) [#8]

I did play around with those, but not much yet. I'll look at them more carefully.